Regarding a measuring device of this type, there is known one comprising a detection part that allows a pair of electrode parts to be brought into contact with a measurement object so as to generate a potential difference to be outputted and further comprising a main body displaying characteristic values pertaining to the pH, ion concentration and the like of the measurement object obtained based on the potential difference as the measurement results. For example, there is a proportional relationship between the potential difference and a logarithm of the ion concentration, and hence it becomes possible to perform an accurate measurement by previously executing a calibration using a reference solution of a known ion concentration before performing the measurement.
By the way, in the calibration performed in the device of this type, since the potential difference outputted from the electrode part is continuously varying for a while after immersing the electrode part into the reference solution, it is necessary to start the calibration after waiting until the variation amount becomes small to be stabilized. Since a timing of starting the calibration is differed according to differences in peripheral environment, concentrations of reference solutions and the like, an unskilled user may start the calibration without taking a sufficient wait time so that the accurate calibration cannot be done in some cases. In order to solve such a problem, there may be used a measuring device that automatically starts the calibration when it is determined by an internal calculation that the potential difference is stable within a tolerance range after a user immerses the electrode part into the reference solution.
In the measuring device having an automatic calibration function by the internal calculation mentioned above, as disclosed in Patent Literature 1, in such a case where the potential difference is not stabilized even after a predetermined time lapse, it is configured to output an error so as to stop the calibration. In other words, if the measuring device having such an automatic calibration function is used, even though the electrode part is immersed into the reference solution the calibration is not always finished but there may result in causing a case, even being rare, where an error is generated so as to stop the calibration.
If a stop of the calibration occurs, since the stability of the potential difference is merely determined by the internal calculation in the measuring device, the user cannot determine as to whether the reference solution is actually unstable or there is a problem in the measuring device itself. If there arises such a doubt, even when the calibration is successful without a problem in the measuring device, the user is apt to consider that a problem may actually arise in the measuring device thereby damaging the reliability of the measuring device.
Meanwhile, as shown in Patent Literature 2 and FIG. 6, in the case of an ion concentration measuring device 100A in which a calibration is performed by a manual operation and stability or instability of a potential difference is informed by a flashing period of an indicator I, it may appear that the problem concerning the reliability as mentioned above can be even solved.
However, even in the measuring device 100A shown in Patent Literature 2, since the flashing period is merely varied, the user cannot understand merely by watching the indicator I or a display screen and the like as to why the measuring device determines that the potential difference is stable. Therefore, there is a first problem that, if an error occurs after starting a calibration so that the calibration is forcibly stopped similarly to the conventional one, the reliability of the indicator I and the measuring device 100A is still lost.
Further, in the measuring device 100A shown in Patent Literature 2, there is a problem also in a measure of a degree of stability used in the internal calculation, in ensuring the reliability of the calibration. Specifically, in this measuring device, the degree of stability Δ as to whether the potential difference is stable is determined by an assessment as to whether a value obtained by dividing a difference between a current potential difference En and a potential difference Em before a predetermined time by a electrode sensibility S ranges within a predetermined fixed threshold. Moreover, since it is known that the potential difference and a logarithm of the ion concentration is proportional, when using concentrations Cn and Cm together with constants Gl and Gu indicative of the lower limit and upper limit of the thresholds corresponding to En and Em and assuming that an equation (Cn=Cm+a) can be described using a variation range “a”, this results in that the determination of the stability is performed in the measuring device under the condition as following.
                                                                                          G                  l                                <                Δ                            =                            ⁢                                                En                  -                  Em                                S                                                                                        =                            ⁢                                                log                  10                                ⁢                                  Cn                  Cm                                                                                                        =                            ⁢                                                                    log                    10                                    ⁡                                      (                                          1                      +                                              a                        Cm                                                              )                                                  <                                  G                  u                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
In other words, it is concluded that the stability is evaluated by a logarithm of a ratio of the ion concentrations indicative of the characteristic values. Herein, assuming that the variation range “a” can be represented as a substantially a constant value obtained by Equation 1 irrespective of the concentration, since the degree of stability Δ is a logarithm value of the concentration, the lower the concentration of Cm, the larger the degree of stability Δ becomes, and conversely the higher the concentration of Cm, the degree of stability Δ is outputted as the smaller value. Therefore, since the threshold is fixed as shown in Equation 1, even if the variation range “a” is substantially equal in the respective ion concentrations during execution of the calibration, it becomes that the stability is determined with a tight criterion in the case of using a reference solution of a low concentration. Conversely, in the case of using a reference solution of a high concentration, it becomes that the stability is determined with a mild criterion.
As described above, in the measuring device of Patent Literature 2, there is a second problem that, if a calibration is performed using reference solutions having a plurality of different characteristic values, the determination criterion of the degree of stability has been varied so that the reliability of the calibration cannot be sufficiently ensured.